1. Field of the Invention
The present invention relates in general to plasma generators and, in particular, to plasma generators utilizing Penning-discharge type configurations using dc discharges and magnetic fields to enhance electron confinement.
2. Description of the Related Art
Orbiting spacecraft travelling through space plasma build up high potentials on their outer surfaces which are often made of insulating materials, due to bombardment by both negative and positive particles. Such potentials are often localized on a particular portion of the spacecraft, which results in different areas of the spacecraft's outer shell being charged to different voltage potentials. For example, in a nonspinning spacecraft in solar orbit, one side faces the sun and one faces darkness. The side of the spacecraft which faces the sun will not become charged, because radiant solar energy causes photoelectrons to be emitted from the spacecraft's surface, and this photoemission compensates for the incident electrons, thereby limiting charge buildup on that side of the spacecraft. The dark side, however, can become charged to very large negative potentials, e.g., -10 kilovolts.
The difference in potential between various areas on the spacecraft's surface can cause an electrical discharge therebetween, which can damage electronic equipment on the spacecraft.
Alternatively, particle bombardment can result in the entire spacecraft being charged to a different potential than space plasma, which could, for example, have an adverse effect on scientific satellites which measure the ambient environment around the spacecraft.
Plasma sources, which produce positive ions and electrons, have been used to discharge the surface of a spacecraft and to clamp the spacecraft to space potential. Such plasma sources discharge the spacecraft because the emitted electrons will be attracted to the positively charged side of the spacecraft while the emitted ions will be attracted to the negatively charged side of the spacecraft, thereby bringing the spacecraft's surfaces to the same potential. In addition, the plasma source clamps the spacecraft frame to space potential because the emitted plasma provides a conductive bridge to the space plasma.
Prior plasma sources have suffered from the disadvantages of being slow to ignite and of providing a low ion emission current, in addition to being relatively large devices In prior hollow-cathode type Penning-discharge devices, the cathodes require constant heating, to effect thermionic emission of electrons, each time the plasma source is to be used. This can result in the power consumption of the plasma source being on the order of 20 watts and several minutes being required to heat the cathode to ignition temperature.
Accordingly, it is the primary object of the present invention to reduce the size, ignition time, and gas and power consumption of a Penning-discharge type plasma source, while achieving large electron and ion currents.